The present invention relates to a method and apparatus for producing hardened formed parts.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
High strength steel sheets which are hot formed and press hardened into formed parts are typically used in the automobile industry for weight reduction and increase in strength in the event of a crash. Hardening of the formed part is realized through cooling, whereby indirect cooling or direct cooling is applicable. Indirect cooling is implemented via cooling channels in the form of bores or slots (shaft cooling) which are arranged in a mold at a defined distance to the molding surface. Coolant, normally water, flows through these channels to dissipate heat, transmitted by the hot formed part to the mold, towards the outside. Direct cooling involves a direct contact of the formed part in the thermoforming mold with the coolant.
Heat transfer and heat dissipation are influenced by contact pressure and a contact between the formed part and the thermoforming mold. The molds are precisely manufactured up to one-hundreds of a millimeter using CNC machines and then surface-treated in an attempt to maintain the gap between the formed part and the mold as small as possible. This has proven difficult in those regions of a formed part that are stretched or have steep grooves because of the resultant presence of air gaps between the formed part and the mold. These air gaps act as insulation, thereby adversely affecting the heat transfer. Air gaps between the formed part and the mold are also encountered as a result of wear.
The cooling process is enhanced by using water as coolant because of its high evaporation enthalpy. Depending on the surface temperature on the formed part, various boiling phenomena can be experienced when the coolant contacts the formed part. When the surface temperature is high, water evaporates and forms on the surface of the formed part a vapor film which has an insulating effect as a consequence of a lesser thermal conductivity compared to the liquid. In the area of film boiling, the formed part thus cools down slower. When the surface to be cooled drops below the so-called Leidenfrost temperature, there is a local and irregularly distributed direct contact across the formed part surface between liquid and formed part. The dissipated heat flow rises in these regions. As soon as the temperature of the coolant drops below the boiling temperature, no evaporation occurs and heat is transferred convectively as the surface of the formed part is completely wetted. The afore-described boiling phenomena or phase states of coolant during cooling could cause local as well as varying and uncontrollable cooldown processes on the formed part. This adversely affects the properties of the formed part and ultimately also impairs product quality.
It would therefore be desirable and advantageous to address prior art shortcomings and to provide more efficient heat transfer and thus improved cooling while allowing adjustment of material characteristic values of formed parts in a reliable and reproducible manner.